Cyclical current reversal for an electrophoretic deposition



Aug. 10, 1965 'r. H. OSTER 3,200,058

CYCLICAL CURRENT REVERSAL FOR AN ELECTROPHORETIC DEPOSITION Filed Aug.23, 1961 OWER SOURCE THOMAS h. OSTER INVENTOR.

A TTORNEKS United States Patent 3 200,058 CYCLICAL CURRENT REVERSAL FGRAN ELEQTROPHORETEC DEPOSETION Thomas H. Oster, Dearboru, Mich, assignorto Ford Motor Company, Dear-born, Mich, a corporation of Delaware FiledAug. 23, 1961, Ser. No. 133,441 5 Claims. (Cl. 204-481) This inventionrelates to the coating of articles by electrophoretic deposition. Moreparticularly, this invention relates to the repeated periodical reversalof the electric current and electric field during the electrophoreticcoating process,

The improvement of the electrophoretic coating process of this inventioncomprises the repeated periodic reversal of the electric field duringthe coating of an article which has been placed in a coating bathcontaining a colloid. Upon reversal of the electrical field, the colloidof the coating bath which has been previously migrating towards thesurface of the article now migrates away from the surface of thatarticle. This reversal of the electric field causes particles which arein the immediate vicinity of the submerged article to move away from thesurface of the article. Charged particles which have been deposited uponthe surface of the article maintain their position on the article. Thus,the deposition is irreversible in the sense that the deposited particleslose their tendency to return into the colloidal dispersionnotwithstanding the reversal of the electrical rfield. When the electricfield is again reversed, the particles will again tend to migrate towardthe article being coated. The areas of the article most sparsely coveredby deposited colloid particles will tend to exert the greatestattraction for the migrating particles since these areas are at thehighest potential; hence, areas not completely covered by colloidalparticles will tend to Ibe coated upon returning the electric held toits initial condition. The foregoing process of migration toward thearticle to be coated and migration away from the article to be coatedwill result in a surface coating of substantially uniform thickness.Further, the coated surface is virtually free firom any gas bubbles thatare generally formed during the coating process.

It should be understood that the use of repeated current reversal isdisclosed in the United States Patent 2,678,909 and is well known in theelectr c-plating industry; however, large and fundamental differencesexist between the former process and the process hereinafter described.It is known to those skilled in the art that an electro-deposit has atendency to redissolve deposited metal in the baths during theapplication of the reverse current, while electroph-oreti-c depositions,to which the process hereinafter described applies, results in anirreversible deposition. Thus, in electroplating metal dissolves .fromthe anode while the piece receives its protective coating. During thereverse cycle the metal dissolves from the piece coated and isredeposited on the anode. The practical consequence of these facts forthe electroplater is that the total plating coulombs must be larger thanthe total reverse or deplating coulombs. Further, it should be notedthat the reason for applying the reverse current in electro-plating isnot applicable to electrophoretic depositing. In electroplating thereversing of the current causes the high point in the deposition to beeroded away from the surface of the material, thereby smoothing thesurface. In electrophoretic deposition this erosion away does not takeplace, The foregoing phenomena is clearly evidenced by FIGURES 2 and 3of the above cited U.S. Patent 2,678,909. Further, the depositing cyclesused in the instant invention and the voltages 32%,858 Patented Aug. 10,1965 used would not be satisfactory in a plating process where metalions are being deposited.

The characteristic advantages of the hereinafter described inventionare:

(1) The coulombs used for cleaning can be equal or even greater than thecoulombs used for electrophoretic deposition.

(2) The cleaning period of one piece can be used as part of the coatingperiod for another piece in the same bath.

(3) The resulting electrophcretic coatings are of superior quality andappearance.

The object of the invention is to provide an electrophoretic process ofcoating whereby a smooth surface resultsr Another object of theinvention is to provide an elec trophoretic process of coating materialswhereby a surface free of gas bubbles results.

Another object of the invention is to provide an electrophoretic coatingprocess whereby an article is coated with paint and the coating is freefrom gas bubbles.

Another object of the invention is to apply cyclical current reversaltechniques to a coating process involving an irreversible deposition.

Another object of the invention is to apply cyclical current techniquesto an electrophoretic coating process.

Other objects of this invention will in part be obvious and will in partappear hereinafter.

In the figures:

FIGURE 1 is a schematic drawing depicting the apparatus used inperforming the cyclical current reversal process of electrophoreticdeposition.

FIGURE 2 is a schematic diagram of another embodiment for accomplishingthe cyclical current reversal process of electrophoretic deposition.

Referring to FIGURE 1, chemically resistant tank 1 contains a coatingbath 5. The articles to be coated 6, 7 are placed upon hangers 9, Thehangers 9 are supported on conductor bars 3. The conductor bars 3 areconnected to the source 2 by a conductor 4. The conductor 4 has a switch8 placed in the circuit in order to initiate .and terminate the coatingprocess. It should be understood that the current source 2 may be adirect current source such as a battery or rectifier combined withsuitable relays, reversing switches and resistancescontrolled by atiming mechanism to reverse the current at predetermined times and varyits value in accordance with the requirements of the articles beingcoated, and the coating bath. The electric potential is above,preferably substantially above, the value at which the electrolysis ofwater occurs. Alternative electrical controls are shown inElectropla't-ing Engineering Handbook, published by Reinhold PublishingCo, A. K. Graham, Editor, pages 578-580, (1955 Each deposition cycle hasa coating period and a cleaning period. The coating period is thatportion of the cycle during which article 6 is being coated. Thecleaning period is that portion of the cycle during which colloidalparticles are migrating away from article 6. The current source 2generates as asymmetrical electrical signal, that is, the duration ofthe electrical signal which causes the coating period of each cycle isgreater than the duration of the signal which causes the cleaning periodof each cycle. This is ordinarily necessary in order that there besuflicient time for colloidal particles to reach the article beingcoated, once the particles in the immediate vicinity of the article havebeen deposited. However, when a bath agitating means is used, it is notnecessary to use an asymmetrical current. Further, when long coating andcleaning periods are used, it is not usually necessary to use anasymmetrical current source.

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The coating bath is a colloidal suspension consisting of colloidalparticles dispersed in a liquid. The colloidal particles hereinafterdescribed carry a negative charge. However, the teaching of thisinvention is not confined to the deposition of negatively chargedparticles. The electrophoretic technique is equally applicable to thedeposition of positively charged colloidal particles. The colloidalparticles, can be dispersed in any of the Well-known electrophoreticvehicles, and the instant invention is not limited to any particularliquid vehicle. The behavior of pigment particles in various liquidvehicles is discussed in the' article, Electrophoretic Behavior ofPigment Suspension, by I-l. Brintzinger, R. Haug and G. Sachs, Farbe andLack, volume 58, pages 10 (published January 1952).

One red primer which has been used as a colloidal particle and which hasbeen electrophoretically deposited as a prime coating on an electricallygrounded steel panel is formulated as follows:

A film-forming material consisting of a styrene-allyl alcohol copolymer63 percent by weight is mixed with linseed fatty. acid 37 percent byweight. This mixture is esterlfied at 500 F. to an acid number of 5 anda maximum viscosity of 2.5 poises, measured when the copolyrner isreducedto 60 percent nonvolatile with xylene. A portion of thisfilm-forming mixture 51 percent by Weight is intimately blended in aroller mill with red oxide pigment 45 percent by weight and linseedfatty acid 4 percent by weight. This blend 38 percent by weight is thenletdown by further blen'ding it with an additional portion of thefilm-forming material of the above composition 50 percent by weight,melamine formaldehyde 11.8 percent by weight, and cobalt naphthanate .2percent by weight.

This paint mixture is then emulsified by the addition of an emulsifingagent which consists of chemically pure concentrated ammonium hydroxide(28 percent strength) 3.5 percent by weight and dernineralized water96.5 percent by .weight. This diluted ammonium hydroxide solution isgradually added until the so-called inversion point is reached, at whichtime the viscosity of the paint mixture drops suddenly after reaching ahigh point. The balance of the diluted ammonium hydroxide is then added.The resulting emulsion is then further refined in a colloid mill until amore stable emulsion is reached.

The paint composition of the above example is then utilized to preparean aqueous coating bath for electropho retically coating an article. Oneexample of the electrophoretic coating process utilizing this paintcomposition is as follows:

Example 1 Two parts of the automotive primer paint composition describedabove is thoroughly mixed with 6 parts of water and A; part ofconcentrated ammonium hydroxide to form a colloidal dispersion to beused as a coating bath. This coating bath has a specific resistance ofapproximately 5.() 10 ohm centimeter.

The foregoing coating bath is placed in the chemically resistant tank 1.The articles to be coated are placed on the support bars 3 by means ofthe hangers 9 and are immersed in the bath. The hangers 9, support bars3, conductor 4'and closed manual switch 8 form an electrical connectionto the current source 2. The current source 2 causes a potentialdifference of. 100 volts to be applied across the articles 6 and 7 beingcoated. This potential difference initially results in article 6 beingpositively charged and article 7 initially being negatively charged. Thepotential difference is maintained for four seconds, during whichnegatively charged colloidal particles are deposited upon the surface ofpositively charged article 6. Following the initial four seconddepositing cycle, the positive 100 volts is applied to article 7 for twoseconds whereupon article '7 becomes more positively charged thanarticle 6 and the negatively charged colloidal 2 particles migrate awayfrom article 6 and are deposited on the article '7. 7

The foregoing cycle is repeated for 70 seconds; thus, 12 four-secondcoating periods and 11 two-second cleaning periods are applied to tirearticles. Following the completion of the twelfth four-second period,the article 6 is removed and another article is placed on hanger 9.-

The above coating process is repeated and then both articles areremoved.

Example 2 Examples 3 and 4 The foregoing examples are applied to acoating bath consisting of automotive primer paint compositionheretofore described mixed with 6 parts of Water 'alfld' flg part ofconcentrated ammonium hydroxide and 19, part of a soap detergent to forma colloidal dispersion. The specific resistance of this coating isapproximately 1.0x 10 ohm centimeter.

' A second embodiment of this invention is illustrated in FIGURE 2. Thearticles '20 to be coated are trans ported by a conveyor 21 to achemically resistant tank 22 containing the coating bath 23. Thearticles 20 are hung from the conveyor 21 by a hook 224, which has anelectrically insulated portion 25 to isolate the article 20 from ground.A contact plate 26 is attached to the paint hook 24 below insulatedportion '25. Upon passing into the coating bath, contact plate 2% makeselectrical contact with one the segments of segmented bus bar 27. Thesegmented bus bar 27 has segments 28 which are connected to the powersource 3%; The segmented bus bar 27 also has segments 2% which areconnected to ground. The paint hooks 24 are so spaced that there arealways at least two articles in the coating baths. The segments of thesegmented bus bar 27 are so spaced'tha-t one article in the coating bathwill be in contact with a segment 28 connected to the power source whilethe other article is in contact with a segment 29 connected to ground.

The speed of the conveyor carrying the articles and the length of eachsegment will control the cycle time. For example, if the articles werecarried by the. conveyor at the rate of 15 feet per minute, the segmentswould have to be 2.5 feet long for a '10 second coating cycle and 2.25feet long for a 9 second cleaning cycle.

. It should be understood that an electrode, which is not an article tobe coated, can be utilized in accordance with the teachings of theinvention.

In order to prevent shorting of the positive and negative segments,suitable insulatorsegments (not shown) are inserted between each segmentof opposite polarity.

If the voltage and current used are high enough to cause extensivearcing, it would be necessary to provide a meansof minimizing thisarcing. For this purpose the segmented bus bar would be fastened in aninsulated trough and covered with a layer of transformer oil. The troughwould be open atthe top so that the contact plate can make goodelectrical connection with the segmented bus bar under, the layer ofoil. 7 The trough would be as long as the segmented bus bar plusadditional s ace for the arm contacts to enter and leave. The ends ofthe trough would be sealed in order to keep the oil at the proper level.a I i r p In experiments performed in accordance Withthe invention, theresulting coated surfaces have been virtually free of any gas bubbles..i a

It is intended that all the matter contained in the above descriptionshall be deemed to be illustrative and not limiting.

I claim:

1. In a method of applying a coating to a conducting surface of anarticle comprising the steps of causing the surface of said article tocontact an aqueous bath having organic film-forming material dispersedtherein, said article serving as a first electrode, causing a secondelectrode to contact said aqueous bath, and causing a direct electriccurrent to flow between said first and second electrodes and throughsaid aqueous bath at an electric potential substantially above the valueat which the electrolysis of water occurs until a coating of saidfilm-forming material is electrically deposited upon said article fromsaid aqueous bath, the improvement which comprises causing thedirect-ion of flow of said direct current between said first and secondelectrodes to be reversed after a significant period of time for apredetermined and significant period of time, and repeating the reversalwhile said article is in contact with said aqueous bath.

2. In a method of applying a coating to a conducting surface of anarticle comprising the steps of causing the surface of said article tocontact an aqueous hath having charged, colloidal coating materialdispersed therein, said material including a film-forming organic resinbinder and a pigment, said article serving as a first electrode, causinga second electrode to contact said aqueous bath at an electric potentialsubstantially above the value at which the electrolysis of water occursuntil a coating of said material is electrophoretically deposited uponsaid article from said aqueous bath, the improvement which com risesreversing the direction of flow of said direct current after asignificant period of time, causing the resulting reversed current toflow for a predetermined and significant period of time, reversing thedirect-ion of said reversed current, repeating such reversals while saidarticle is in contact with said aqueous bath.

3. In a method of applying a coating to a conducting surface of anarticle comprising the steps of causing the surface of said article tocontact an aqueous bath having charged particles of organic coatingmaterial dispersed therein, said article serving as a first electrode,causing a second electrode to contact said aqueous bath, and cansing adirect electric current to flow between said first and second electrodesand through said aqueous bath at an electric potential substantiallyabove the value at which the electrolysis of water occurs until acoating of said coating material is electrophoretically deposited uponsaid first electrode from said aqueous bath, the improvement whichcomprises causing the direction of flow of said direct current betweensaid first and second electrodes to be reversed after a predeterminedand significant period of time for a predetermined and significantperiod of time and repeating such reversals after comparablepredetermined periods of time while said electrodes are in contact withsaid bath until said first electrode and said second electrode arecoated with an electrically irreversible coating of said organic coatingmaterial.

4. In a method of applying paint to a metallic article comprising thesteps of causing the surface of said article to be immersed in anaqueous bath having a paint formulation comprising charged particles ofa film-forming, organic resin dispersed therein, causing a secondmetallic article to contact said aqueous bath, and causing a directelectric current .to fiow between said first and second articles andthrough said aqueous bath at an electric potential substantially abovethe value at which the electrolysis of water occurs until a Waterinsoluble coating of said resin is electrically deposited upon saidfirst article, the improvement which comprises reversing the directionof flow of said direct electric current between said first article andsaid second article after a predetermined andv significant period oftime for a second significant period of time, reversing said directionof flow after said second period and repeating the reversal of flowafter comparable periods of time while said first and second articlesare in contact with said bath.

5. In a method of applying paint to a metallic article comprising thesteps of causing the surface of said article to be immersed in anaqueous bath having a paint formulation comprising charged particles ofa film-forming, organic resin dispersed therein, said articles servingas a first electrode, causing a second electrode to contact said aqueousbath, and causing a direct electric current to flow between said firstand second electrodes and through said aqueous bath at an electricpotential substantially above the value at which the electrolysis ofwater occurs until a water insoluble coating of said paint formulationis electrolphoretically deposited upon said first electrode, theimprovement which comprises reversing the direction of flow of saiddirect electric current between said first and second electrodes after apredetermined and significant period of time for a predetermined andsignificant period of time of different duration, reversing saiddirection of flow after said period of different duration, and repeatingthe reversal of flow after corresponding periods of time while saidfirst and said second electrodes are in contact with said bath.

References Cited by the Examiner UNITED STATES PATENTS 1,769,659 7/30Williams 204--182 2,314,604 3/43 Van der Horst 204-26 2,678,909 5/54Jernstedt 204-41 FOREIGN PATENTS 482,548 3/38 Great Britain.

WINSTON A. DOUGLAS, Primary Examiner. MURRAY TILLMAN, JOHN R. SPECK,Examiners.

1. IN A METHOD OF APPLYING A COATING TO A CONDUCTING SURFACE OF ANARTICLE COMPRISING THE STEPS OF CAUSING THE SURFACE OF SAID ARTICLE TOCONTACT AN AQUEOUS BATH HAVING ORGANIC FILM-FORMING MATERIAL DISPERSEDTHEREIN, SAID ARTICLE SERVING AS A FIRST ELECTRODE, CAUSING A SECONDELECTRODE TO CONTACT SAID AQUEOUS BATH, AND CAUSING A DIRECT ELECTRICCURRENT TO FLOW BETWEEN SAID FIRST AND SECOND ELECTRODES AND THROUGHSAID AQUEOUS BATH AT AN ELECTRIC POTENTIAL SUBSTANTIALLY ABOVE THE VALUEAT WHICH THE ELECTROLYSIS OF WATER OCCURS UNTIL A COATING OF SAIDFILM-FORMING MATERIAL IS ELECTRICALLY DEPOSITED UPON SAID ARTICLE FROMSAID AQUEOUS BATH, THE IMPROVEMENT WHICH COMPRISES CAUSING THE DIRECTIONOF FLOW OF SAID DIRECT CURRENT BETWEEN SAID FIRST AND SECOND ELECTRODESTO BE REVERSED AFTER A SIGNIFICANT PERIOD OF TIME FOR A PREDETERMINEDAND SIGNIFICANT PERIOD OF TIME, AND REPEATING THE REVERSAL WHILE SAIDARTICLE IS IN CONTACT WITH SAID AQUEOUS BATH.